1. Field of the Invention
This invention relates to the preparation of self-crosslinking copolymers derived from blocked isopropenyl-alpha, alpha-dimethylbenzyl isocyanate and hydroxy group containing olefinically unsaturated monomers. Self-crosslinking copolymers are useful as self-curing resins in coatings, adhesives, and moldings.
2. Description of the Related Art
In general, commonly used urethane coatings systems are of the two-component or one-component types.
A typical two-component paint comprises, for example, a polyisocyanate material and a polyhydroxy material, which upon curing, can form films with excellent physical properties and resistance properties. Two component systems are used in high performance coatings.
A typical one-component paint comprises, for example, a blocked polyisocyanate and a polyhydroxy material. Because the isocyanate groups are blocked, one component systems generally have a longer shelf life than two-component systems containing free, hydroxy-reactive isocyanate groups. When desired, the one-component systems may be activated by, for example, heat, to deblock the blocked polyisocyanate and promote crosslinking.
Japanese Patent No. 63-186,722 discloses a coatings system wherein the blocked isocyanate and hydroxy groups are part of the same polymer backbone, thereby producing a self-crosslinking type one-component system. Similar ethanol-blocked copolymers are disclosed in JP 1-087,606.
U.S. Pat. No. 5,116,930 discloses another approach to self-crosslinking resins. This approach is accomplished by hydroxyfunctionalization of a polyiso-cyanate polymer. It is stated therein in column 1 and in Comparative Example 3, Column 9, that when a self-curing resin is produced by copolymerizing an oxime-blocked isocyanate-containing free radical-polymerizable monomer and a hydroxy-containing free radical-polymerizable monomer, the blocking agent may undergo dissociation during polymerization, disadvantageously leading to gelation, as it does in the comparative Example 3 using xylenes as the solvent. Further, during our efforts to prepare these polymers, we have ourselves observed, on numerous occasions, the setting up of the contents of the reactor into a crosslinked gel destroying the reactor used in the process. A solution for the gelation problem has been described in the copending application submitted concurrently herewith entitled "N-Hydroxysuccinimide-Blocked Isopropenyl-alpha, alpha-Dimethylbenzyl Isocyanate and Self-Crosslinking Copolymers Thereof", American Cyanamid Case Ser. No. 07/963,778. Another solution to the gelation problem comprising using a polar solvent in the preparation of the self-crosslinking copolymers is described herein.
It is therefore an object of this invention to provide an improved process for preparing gel-free self-cross-linking copolymers wherein the improvement comprises carrying out the polymerization in a polar organic solvent which retards the dissociation of the blocking groups, thereby preventing premature gelation.